This invention relates generally to highlight color imaging and more particularly to a printing apparatus and method for forming one black and two color images,
In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a charge retentive surface such as a photoconductive member by first uniformly charging the charge retentive surface. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, not only are the charged (i.e., unexposed) areas developed with toner but the discharged (i.e., fully exposed) images are also developed. Thus, the charge retentive surface contains three voltage levels which correspond to two image areas and to a background voltage area. One of the image areas corresponds to non-exposed (i.e. charged) areas of the photoreceptor, as in the case of conventional xerography, while the other image areas correspond to fully exposed (i.e., discharged) areas of the photoreceptor.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by Charged Area Development, i.e. CAD) remains at or near the fully charged photoreceptor potential represented by V.sub.cad or V.sub.ddp. The other images are formed by discharging the photoreceptor to its residual potential, i.e.V.sub.dad or V.sub.c (typically 100 v) which corresponds to discharged area images that are subsequently developed by Discharged Area Development (DAD). The background areas are formed by discharging the photoreceptor to reduce its potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.white or V.sub.w. The CAD, V.sub.bb developer is typically biased about 100 v closer to V.sub.cad than V.sub.white is to V.sub.cad, resulting in a V.sub.bb of about 600 volts, and the DAD developer system is biased about 100 v closer to V.sub.dad than V.sub.white is to V.sub.dad resulting in a V.sub.cb of about 400 volts.
As developed, the composite tri-level image initially consists of both positive and negative toners. To enable conventional corona transfer, it is necessary to first convert the entire image to the same polarity. This must be done without overcharging the toner that already has the correct polarity for transfer. If the amount of charge on the toner becomes excessive, normal transfer will be impaired and the coulomb forces may cause toner disturbances in the developed image. On the other hand, if the toner whose polarity is being reversed is not charged sufficiently its transfer efficiency will be poor and the transferred image will be unsatisfactory.
In the past few years there has been interest in extending trilevel xerography to black plus two colors. In U.S. Pat. No. 5,049,949 granted to Parker et al on Sep. 17, 1991 describes how this can be done by creating, and developing, a conventional two color trilevel image, followed by a second exposure, and development of a second DAD image of a third color. The advantage of this method is that it produces a three color image with one less exposure than by any other means. The disadvantage is that, while the first two images are perfectly registered with one another, the third is subject to the normal misregistration associated with a second exposure.
More recently, in U.S. patent applications Ser. Nos. 07/987,886, now U.S. Pat. No. 5,444,463, and 07/987,885, now U.S. Pat. No. 5,373,313, filed on Dec. 9, 1992 in the names of Gregory J. Kovacs et al and Kovacs et al, respectively and assigned to the same assignee as the instant application disclose a way to create a three color, multiwavelength exposure, perfectly registered, trilevel latent image using a single exposure step. This technique relies on a dual beam laser where one beam has a wavelength .lambda.1, and the other .lambda.2, and where both are focussed adjacent to each other on the photoreceptor. The photoreceptor consists of two photosensitive layers of approximately equal dielectric thickness, one layer of which responds to .lambda.1, but not .lambda.2, and the other to .lambda.2, but not .lambda.1.
FIGS. 1 and 2 illustrate how the multiwavelength exposure method works. Here, the upper portion shows the voltage profile of the two layered photoreceptor after it has been exposed by .lambda.1 or .lambda.2, .lambda.1+.lambda.2 combined, or neither. Immediately below the voltage profile is the corresponding cross section of a two layer photoreceptor where the shaded areas denote portions that remain charged after exposure.
Wherever only light of wavelength .lambda.2 strikes the photoreceptor, the top layer, for example, of photoreceptor is fully discharged leaving the photoreceptor surface potential at .about.V.sub.0/2. This is the background reference (V.sub.white) component of the trilevel latent image. In regions where both beams are off, the photoreceptor remains charged to V.sub.0 forming the first color, Charged Area Developed (CAD) portion of the three color trilevel latent image. The second color, Discharge Area Developed (DAD) latent image is formed wherever both the upper and lower layers of photoreceptor are discharged to V.sub.residual by both beams acting in unison. The third color, Discharge Area Developed (DAD) latent image, is formed by a .lambda.1 imagewise exposure which leaves the photoreceptor charged to .about.V.sub.0/2.
At this point, the photoreceptor voltage profile contains a first color CAD image and a second color DAD image above, and below respectively, the reference voltage V.sub.white, but because the third color DAD latent image is also at the V.sub.white, it is electrostatically indistinguishable from background to the first and second development systems. As shown in FIG. 2, after the first color CAD and second color DAD images have been developed, the photoreceptor is then flood exposed by light of wavelength .lambda.2 or some other suitable wavelength which will discharge layer 2 but not layer 1. This additional exposure has no effect on the potential in the background regions that have been previously exposed by .lambda.2, but does complete the discharge in regions that were exposed by .lambda.1. This makes the third color DAD image electrostatically visible or distinguishable for development at the second DAD station. The toner used to develop the CAD and first DAD images are opaque to light at the wavelength of the flood exposure in order to avoid developing additional voltage offsets.
Although the CAD image and the first DAD image can be developed by conventional trilevel Conductive Magnetic Brush (CMB) techniques, development of the second DAD image presents a problem. CMB relies on contact development which does not simply develop to neutralization and stop, but rather, reaches a state of equilibrium in which the deposition rate and scavenging rate come into balance. Because the toners in both DAD development stations have a common polarity, a third contact development step will interact with the DAD image already developed and cause a hue shift. Scavenged toner will also color contaminate the third developer, and eventually produce a hue shift in the second DAD image. Clearly, contact development is unacceptable at the third development station.
Unfortunately, non-contact, powder cloud-like development systems typically respond to electrostatic edges of .about.30 volts or more. In addition, CMB development systems require cleaning fields on the order of .about.100 volts to suppress spurious background development. Hence, even if the CMB development completely neutralized the development field with respect to the bias, the developed image will still be offset from background .about.100 volts. The shaded areas in FIG. 3 denote the neutralized CAD and first DAD image after development. Here, assuming that the third non-contact development station is biased to also produce a .about.100 volt cleaning field, the voltage offset at the edges between the CAD, and second DAD image would be .about.200 volts, and therefore halos of the third color would be developed around the CAD image.